Jasim U. Ahmad

Automated CFD Parameter Studies on Distributed Parallel Computers

The objective of the current work is to build a prototype software system which will automated the process of running CFD jobs on Information Power Grid (IPG) resources. This system should remove the need for user monitoring and intervention of every single CFD job. It should enable the use of many different computers to populate a massive run matrix in the shortest time possible. Such a software system has been developed, and is known as the AeroDB script system. The approach taken for the development of AeroDB was to build several discrete modules. These include a database, a job-launcher module, a run-manager module to monitor each individual job, and a web-based user portal for monitoring of the progress of the parameter study. The details of the design of AeroDB are presented in the following section. The following section provides the results of a parameter study which was performed using AeroDB for the analysis of a reusable launch vehicle (RLV). The paper concludes with a section on the lessons learned in this effort, and ideas for future work in this area.

Navier-Stokes Simulation of Air-Conditioning Facility of a Large Modern Computer Room

A 3-D full Navier-Stokes simulation of a large scale computing facility at NASA Ames Research center was carried out to assess the adequacy of the existing air handling and conditioning system. The flow simulation of this modern facility was modeled with a viscous, compressible flow solver code OVERFLOW-2 with low Mach number pre-conditioning. A script was created to automate geometry modeling, grid generation, and flow solver input preparation. A new set of air-conditioning boundary conditions was developed and added to the flow solver. Detailed flow visualization was performed to show temperature distribution, air-flow streamlines and velocities in the computer room.Copyright

Automated CFD Database Generation for a 2nd Generation Glide-Back-Booster

A new software tool, AeroDB, is used to compute thousands of Euler and Navier-Stokes solutions for a 2nd generation glide-back booster in one week. The solution process exploits a common job-submission grid environment using 13 computers located at 4 different geographical sites. Process automation and web-based access to the database greatly reduces the user workload, removing much of the tedium and tendency for user input errors. The database consists of forces, moments, and solution files obtained by varying the Mach number, angle of attack, and sideslip angle. The forces and moments compare well with experimental data. Stability derivatives are also computed using a monotone cubic spline procedure. Flow visualization and three-dimensional surface plots are used to interpret and characterize the nature of computed flow fields.

Automated Euler and Navier-Stokes Database Generation for a Glide-Back Booster

AeroDB is a database generation framework that is used to compute thousands of Euler and Navier-Stokes solutions for a 2nd generation glide-back booster in one week using the NASA Information Power Grid. Process automation and web-based access is used to greatly simplify and reduce the user workload. The solutions are validated with experimental data, and stability derivatives are computed using a monotone cubic-spline procedure. Flow visualization and three-dimensional surface plots are used to interpret and characterize the nature of computed flow fields.

High-Order Accurate CFD/CSD Simulation of the UH-60 Rotor in Forward Flight

Time-dependent Reynolds-averaged Navier-Stokes simulations for a highly flexible aeroelastic UH-60A rotor have been carried out for forward flight. The fluid structure interaction is accomplished by loosely coupling the OVERFLOW Computational Fluid Dynamics (CFD) code with the helicopter comprehensive code CAMRAD II. The latter includes Computational Structural Dynamics (CSD) with rotor trim capability. Computations are performed using spatial differences of 5 th -order for inviscid fluxes, 2 nd order accurate viscous fluxes, and 2 nd -order time accuracy. Dual time-stepping accuracy is examined and recommendations provided for an appropriate time step and number of subiterations. The predicted sectional normal forces and pitching moments are compared with flight-test data for high-speed and low-speed level-flight conditions. The low-speed case has significant Blade Vortex Interaction (BVI). The current simulations demonstrate a factor of two improvement in state-of-the-art airloads prediction accuracy for CFD/CSD simulations. This is attributed to the use of improved spatial accuracy. I. Introduction OMPUTATIONAL Fluid Dynamics (CFD) simulation of rotorcraft flow fields using the Reynolds-averaged Navier-Stokes (RANS) equations is a challenging multidisciplinary problem. This is primarily due to the need to model the aeroelastic interaction of the fluid dynamics with highly flexible rotor blades. A successful aerodynamic simulation of a rotor/fuselage system requires the modeling of unsteady three-dimensional flows that may include transonic shocks, dynamic stall with boundary layer separation, complex vortical wakes, blade/wake and wake/wake interactions, and body motions. Moreover, a stable and robust method to couple the CFD with a Computational Structural Dynamics (CSD) model is required. Consequently, the aerodynamic and aeroacoustic performance prediction for rotorcraft lags compared to its fixed-wing counterpart. The helicopter industry often uses comprehensive rotor codes as part of the design process. These comprehensive codes typically include a simplified linear aerodynamic model, a CSD method for flexible rotor blades, and a trim algorithm. However, this very efficient design tool lacks the high fidelity aerodynamics needed for nonlinear rotorcraft flows. It is common at NASA and in the rotorcraft industry to carry out a parameter sweep of rotorcraft simulations with a less computationally costly comprehensive code, such as CAMRAD II, 1 when linear

Comparison of Computed and Measured Vortex Evolution for a UH-60A Rotor in Forward Flight

A Computational Fluid Dynamics (CFD) simulation using the Navier-Stokes equations was performed to determine the evolutionary and dynamical characteristics of the vortex flowfield for a highly flexible aeroelastic UH-60A rotor in forward flight. The experimental wake data were acquired using Particle Image Velocimetry (PIV) during a test of the full-scale UH-60A rotor in the National Full-Scale Aerodynamics Complex 40- by 80-Foot Wind Tunnel. The PIV measurements were made in a stationary cross-flow plane at 90 deg rotor azimuth. The CFD simulation was performed using the OVERFLOW CFD solver loosely coupled with the rotorcraft comprehensive code CAMRAD II. Characteristics of vortices captured in the PIV plane from different blades are compared with CFD calculations. The blade airloads were calculated using two different turbulence models. A limited spatial, temporal, and CFD/comprehensive-code coupling sensitivity analysis was performed in order to verify the unsteady helicopter simulations with a moving rotor grid system.

Visualization and Analysis of Vortex Features in Helicopter Rotor Wakes

Vortex interactions in helicopter rotor wakes are visualized using enhanced flow visualization techniques, which in turn play a crucial role in understanding the dynamics of these complex flows. Iso-surfaces are clipped in a 3D volume to reduce visual clutter. Flow textures are improved to highlight vortical flow structure and vortex-wake interactions. These visualization techniques are applied to three Computational Fluid Dynamics (CFD) simulations of helicopter flow fields. The presented techniques provide a good depiction of rotor tip vortices in these flows.

Navier-Stokes Simulation of Local Winds Over the Earth’s Topography

A numerical approach is described that simplifies and automates the CFD solution process so that Earth scientists can utilize high-resolution Navier-Stokes flow solvers as a research tool to investigate wind events on the Earth’s surface. The current approach utilizes the OVERFLOW-2 structured overset RANS code. A genetic algorithm is used to obtain an optimal multi-zone overset grid system that reduces the grid size and simulation time by maintaining high resolution over high-gradient land regions and lower resolution over low-gradient water regions. Flow simulations are presented that include flow separation and reattachment over mountainous terrain for coastal islands in Alaska (USA) and British Columbia (Canada).

Visualization and Quantification of Rotor Tip Vortices in Helicopter Flows

This paper presents an automated approach for effective extraction, visualization, and quantification of vortex core radii from the Navier-Stokes simulations of a UH-60A rotor in forward flight. We adopt a scaled Q-criterion to determine vortex regions and then perform vortex core profiling in these regions to calculate vortex core radii. This method provides an efficient way of visualizing and quantifying the blade tip vortices. Moreover, the vortices radii are displayed graphically in a plane.